1. Field of the Invention
The invention relates to a method of forming a film of a conductive material that is used for an interconnection layer in a semiconductor device.
2. Description of the Related Art
A conductive material such as aluminum is generally used for an interconnection material in a semiconductor device, and a physical vapor deposition (PVD) process, such as sputtering or evaporation, is used for the formation of an interconnection of conductive material in a semiconductor device. It is usual to form such an interconnection of conductive material in a one-step process.
In recently developed semiconductor devices, which have a multi-layer interconnection construction and a progressively refined structure, thinner interconnection layers are required because it is necessary to reduce the level difference between uneven surfaces and facilitate etching during a patterning operation. Even under such circumstances, the film of a formed interconnection layer should be in a continuous state to ensure electrical continuity therethrough.
When a film of a conductive material, such as aluminum, used for an interconnection layer in a semiconductor device is deposited on a substrate by a PVD process, the amount of the conductive material deposited on surfaces having a certain angle to the horizontal plane of the substrate, particularly the side walls of via-holes that are opened in an insulating film and have a wall surface perpendicular to the horizontal plane of the substrate, becomes smaller compared with the amount of conductive material deposited on horizontal surfaces, such as the top surface of the insulating film and the bottom surfaces of the via-holes. For example, on a side wall of a via-hole, the amount of deposited material is experimentally determined to be in the order of a quarter of that deposited on the top surface of an insulating layer and the bottom surface of the via-hole. Consequently, the thickness of the material deposited on the side walls of the via-holes is different from the thickness of the material deposited on the top surface of the insulating layer and the bottom surfaces of the via-holes.
A film of a metallic conductive material such as aluminum is grown in a nucleus growth mode during the deposition by a PVD process. In this nucleus growth mode, atoms of a material to be deposited, such as aluminum, that have been emitted into a vacuum, first reach the surface of a substrate, release thermal energy while migrating thereon, and then form smaller nuclei. The preferential sites for forming the nuclei are provided by atoms of gas adsorbed on the surface of the substrate and lattice defects. When atoms in excess of a critical number agglomerate around the nucleus, these atoms form a larger, stable nucleus. The stable nuclei subsequently consolidate with each other like droplets, and when the deposition proceeds further, a network state occurs, and eventually a continuous film on the substrate is formed. Thus, the continuous film is not formed when the deposition is terminated in spite of a lack of depositing material atoms.
In the situation of increasing development of thinner interconnection layers, films formed on the side walls of via-holes are prone to be discontinuous, particularly when the deposition of a conductive material is finished when there is a lack of atoms deposited on the side walls of the via-holes, on which the deposited film in particular tends to be quite thin. Also in regions other than via-holes, the formed film can be in a network state, or it can have a violently rough surface even if it is continuous, causing problems with the patterning of the film.
Few techniques for forming a film of a conductive material used for an interconnection layer in a multi-layer semiconductor device using two step physical vapor deposition of the material are known in the art.
JP3-240945(A) discloses the formation of a metallic film used for a conducting layer on a substrate by an evaporation or sputtering process in a two-step manner. In the first step, a metallic material is deposited at a substrate temperature not greater than one third of the melting temperature, expressed in absolute temperature, of the deposited material, until a relatively thin film having full or nearly full continuity in the texture thereof is formed (said continuity is represented as "structure with no openings (or gaps)" in the document), and in the second step, the material is deposited onto the film previously formed at a substrate temperature not lower than one half of the melting temperature, expressed in absolute temperature, of the deposited material. Thus, the film initially deposited on a substrate according to the method disclosed in JP3-240945(A) already has full or nearly full continuity.
Further, to the applicant's knowledge, it is not known to interpose an annealing operation in the course of the formation of a continuous film of a conductive material by a PVD process, as in the present invention.